Positive electrode for nonaqueous electrolyte secondary batteries, and nonaqueous electrolyte secondary battery

ABSTRACT

A positive electrode for nonaqueous electrolyte secondary batteries having a positive electrode current collector and a positive electrode mixture layer that is formed on the positive electrode current collector. The positive electrode for nonaqueous electrolyte secondary batteries is: the positive electrode mixture layer comprises a first mixture layer that contains a positive electrode active material and a reaction inhibitor which inhibits a thermal reaction between the positive electrode active material and a nonaqueous electrolyte, and a second mixture layer that contains the positive electrode active material; the positive electrode is obtained by sequentially laminating the positive electrode current collector, the first and the second mixture layer in this order; and the concentration of the reaction inhibitor contained in the positive electrode mixture layer is high in the vicinity of the positive electrode current collector in comparison to that in the surface layer portion of the positive electrode mixture layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.14/773,940, filed on Sep. 9, 2015, which is a Continuation ofInternational Application No. PCT/JP2014/001373 filed on Mar. 11, 2014which is based upon and claims the benefit of priority from the priorJapanese Patent Application No. 2013-047846, filed on Mar. 11, 2013, theentire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a positive electrode for a non-aqueouselectrolyte secondary battery and a non-aqueous electrolyte secondarybattery using the same.

BACKGROUND ART

In a non-aqueous electrolyte secondary battery, when an internal shortcircuit occurs, it causes a large current in the battery, and as aresult sometimes elevates the temperature inside the battery. Thistemperature elevation may induce a reaction between the positiveelectrode active material and the non-aqueous electrolytic solution. Inparticular, an internal short circuit due to a stress applied from theoutside to a battery in a fully charged state discharges a large energyalmost instantaneously to elevate the battery temperature, and thistemperature elevation induces the reaction between the positiveelectrode active material and the non-aqueous electrolytic solution. Ifthis reaction cannot be suppressed, the rapid elevation of thetemperature is likely to occur.

Here, examples of the cause for an internal short circuit includepenetration with an object having a sharp tip (e.g., a nail), thecollapse of a battery due to crushing, and an impact due to falling. Inthe case of an internal short circuit due to penetration with a sharpobject, a large amount of Joule heat is generated near the collector,and the heat induces the reaction between the positive electrode activematerial and the non-aqueous electrolytic solution, which is likely tocause the rapid elevation of the battery temperature. In PatentLiterature 1, it is disclosed to suppress the reaction between thepositive electrode active material and the non-aqueous electrolyticsolution by blending a solid flame retardant agent in the positiveelectrode mixture or the negative electrode mixture. Further, in PatentLiterature 2, it is disclosed to suppress the combustion of thebattery-constituting materials by disposing a flame retardant agentlayer containing a flame retardant agent on either surface of thepositive electrode, the negative electrode or the separator.

CITATION LIST Patent Literature Patent Literature 1: Japanese PatentLaid-Open Publication No. 2009-16106

Patent Literature 2: International Publication No. WO 2010-101180

SUMMARY OF INVENTION Technical Problem

However, in the case of the technique disclosed in Patent Literature 1,in order to suppress the rapid temperature elevation near the collector,the proportion of the flame retardant agent in the mixture is increasedwhich results in deterioration of battery characteristics such as adeteriorated capacity and deteriorated input-output characteristics.Further, in the case of the technique disclosed in Patent Literature 2,the deterioration of battery characteristics occurs because a flameretardant agent layer is not present near the collector, which providesno flame retardant effect against large heat generation in thecollector, and instead the flame retardant agent layer is present in thesurface layer side of the positive electrode and the negative electrode.

It is an object of the present invention to provide a positive electrodefor a non-aqueous electrolyte secondary battery which suppresses thereaction between the positive electrode active material and thenon-aqueous electrolytic solution, and has excellent input-outputcharacteristics, and a non-aqueous electrolyte secondary battery usingthe same.

Solution to Problem

A positive electrode for a non-aqueous electrolyte secondary batteryaccording to the present invention comprises a positive electrodecurrent collector and a positive electrode mixture layer formed on thepositive electrode current collector, the positive electrode mixturelayer comprises a first mixture layer containing a positive electrodeactive material and a reaction inhibitor to suppress a thermal reactionbetween the positive electrode active material and a non-aqueouselectrolyte, and a second mixture layer comprising the positiveelectrode active material, and the positive electrode is formed of thepositive electrode current collector, the first mixture layer and thesecond mixture layer stacked in this order, wherein a concentration ofthe reaction inhibitor contained in the positive electrode mixture layernear the positive electrode current collector is higher than that in asurface layer portion of the positive electrode mixture layer.

Advantageous Effects of Invention

The positive electrode for a non-aqueous electrolyte secondary batteryaccording to the present invention and the non-aqueous electrolytesecondary battery using the same suppress the reaction between thepositive electrode active material and the non-aqueous electrolyticsolution, and have excellent input-output characteristics.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partially sectional view of an example of a positiveelectrode for a non-aqueous electrolyte secondary battery according toan embodiment of the present invention.

FIG. 2 is a perspective view showing an example of a non-aqueouselectrolyte secondary battery according to an embodiment of the presentinvention cutaway in a longitudinal direction.

DESCRIPTION OF EMBODIMENT

Hereinafter, an embodiment according to the present invention will bedescribed in detail. A non-aqueous electrolyte secondary batteryaccording to the embodiment of the present invention, for example, has aconstitution in which an electrode body and a non-aqueous electrolytesolution, which is a liquid non-aqueous electrolyte, are contained in abattery outer can, the electrode body being formed of a positiveelectrode and a negative electrode wound with a separator interposedtherebetween, or alternatively positive electrodes and negativeelectrodes alternately stacked with separators interposed therebetween.Each component of the non-aqueous electrolyte secondary battery will bedescribed in detail below.

[Positive Electrode]

FIG. 1 is a partially sectional view of a positive electrode 10. Thepositive electrode 10 is constituted of a positive electrode currentcollector 20 which is metal foil or the like and a positive electrodemixture layer 21 formed on the positive electrode current collector 20.For the positive electrode current collector 20, there is used a foil ofa metal which is stable in the potential range of the positiveelectrode, a film on which a metal which is stable in the potentialrange of the positive electrode is disposed as a surface layer, or thelike. Aluminum is suitable for use as the metal that is stable in thepotential range of the positive electrode. Further, the positiveelectrode mixture layer 21 is constituted of a first mixture layer 22formed on the positive electrode current collector 20 and containing areaction inhibitor, and a second mixture layer 32 formed furtherthereon.

The first mixture layer 22 is a layer which contains an electricallyconductive material 26, a binder 28, a reaction inhibitor 30 and thelike in addition to a positive electrode active material 24, and isobtained by mixing these materials in a suitable solvent, applying theresultant mixture to the positive electrode current collector 20, dryingthe applied material, and then rolling the dried material.

Further, the second mixture layer 32 is a layer which contains anelectrically conductive material 26, a binder 28 and the like inaddition to a positive electrode active material 24, and is obtained bymixing these materials in a suitable solvent, applying the resultantmixture to the positive electrode current collector 20, drying theapplied material, and then rolling the dried material. For the positiveelectrode active material 24, the electrically conductive material 26and the binder 28 used for the second mixture layer 32, the samematerials as those used for the first mixture layer 22 can be used.

For the positive electrode active material 24, there may be a transitionmetal oxide containing an alkali metal element, for example, in aparticulate shape, or a transition metal oxide in which a portion of thetransition metal element contained in the above-described transitionmetal oxide has been substituted with a different kind of element. Thealkali metal element may include, for example, lithium (Li) and sodium(Na). Among these alkali metal elements, lithium is preferably used. Forthe transition metal element, there may be used at least one transitionmetal element selected from the group consisting of scandium (Sc),manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu),yttrium (Y) and the like. Among these transition metal elements, Mn, Co,Ni or the like is preferably used. For the different kind of element,there may be used at least one different kind of element selected fromthe group consisting of magnesium (Mg), aluminum (Al), zirconium (Zr),tungsten (W), molybdenum (Mo), lead (Pb), antimony (Sb), boron (B) andthe like. Among these different kinds of elements, Mg, Al, Zr, W or thelike is preferably used.

Specific examples of such a positive electrode active material 24 mayinclude LiCoO₂, LiNiO₂, LiMn₂O₄, LiMnO₂, LiNi_(1−y)Co_(y)O₂, (0<y<1),LiNi_(1−y−z)Co_(y)Mn_(z)O₂, (0<y+z<1), and LiFePO₄ as thelithium-containing transition metal oxides in which lithium is used asthe alkali metal element. For the positive electrode active material 24,these materials may be used singly or in combinations of two or morethereof.

The electrically conductive material 26 is a powder, a particle or thelike having electrical conductivity and used in order to enhance theelectron conductivity of the positive electrode mixture layer 21. Forthe electrically conductive material 26, there is used a carbonmaterial, a metal powder, an organic material or the like havingelectrical conductivity. Specifically, the electrically conductivematerial 26 includes acetylene black, Ketjen black, graphite and thelike as the carbon materials; aluminum and the like as the metalpowders; potassium titanate, titanium oxide and the like as metaloxides; and phenylene derivatives and the like as the organic materials.These electrically conductive materials 26 may be used singly or incombinations of two or more thereof.

The binder 28 is a polymer having a particulate shape or a networkstructure and used in order to keep a good contact state between thepositive electrode active material 24 in a particulate shape and theelectrically conductive material 26 in a powder or particulate shape,and to enhance bindability of the positive electrode active material 24and the like to the surface of the positive electrode current collector20. For the binder 28, there may be used a fluorine-containing polymer,an elastomeric polymer or the like. Specifically, the binder 28 includespolytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), modifiedmaterials thereof or the like as the fluorine-containing polymers; andethylene-propylene-isoprene copolymer, ethylene-propylene-butadienecopolymer and the like as the elastomeric polymers. The binder 28 may beused together with a thickener such as carboxymethyl cellulose (CMC),polyethylene oxide (PEO) or the like.

The reaction inhibitor 30 is a powder or particles present in the firstmixture layer 22 and sparingly soluble in the non-aqueous electrolyticsolution, and has a function to suppress the reaction between thepositive electrode active material and the non-aqueous electrolyticsolution which is caused in chains by Joul heat generated in thepositive electrode current collector 20. The reaction inhibitor 30preferably coexists with the positive electrode active material 24,i.e., is fixed near the positive electrode active material 24 to remainin the positive electrode mixture layer 21. As such a reaction inhibitor30 it is possible to use a fluoride, a phosphate ester compound, amelamine-acid salt or the like. More specifically, as a fluoride it ispossible to use lithium fluoride (LiF), aluminum fluoride (AlF), as aphosphate ester compound it is possible to use an aromatic condensedphosphate ester, and as a melamine-acid salt it is possible to usemelamine pyrophosphate, melamine sulfate, melamine polyphosphate,melamine cyanurate and melamine borate. In addition to these substances,any substance can be used as long as it is a substance which suppressesthe reaction between the positive electrode active material and thenon-aqueous electrolytic solution, such as one generally referred to asa flame retardant agent in a non-aqueous electrolyte secondary battery,and can be fixed near the positive electrode active material 24.

FIG. 2 is a perspective view showing a non-aqueous electrolyte secondarybattery according to an embodiment of the present invention cutaway in alongitudinal direction. Here a situation is illustrated in which a woundelectrode body 46 formed by winding a positive electrode 10 and anegative electrode 42 with a separator 44 interposed therebetween isaccommodated in a battery outer can 50. In FIG. 2, when a nail or thelike having a sharp tip, not shown in FIG. 2, penetrates perpendicularlyto the side surface of the battery outer can 50, the nail penetrates thewound electrode body 46 formed of the negative electrode 42, theseparator 44 and the positive electrode 10 stacked in this order, tocause an internal short circuit. As such an internal short circuit,there exist a short circuit between a positive electrode currentcollector 20 and the negative electrode 42 and a short circuit betweenthe surface portion of a positive electrode mixture layer 21 and thenegative electrode 42, and the former short circuit generates a largeramount of heat due to a short circuit. This is considered to be becausea large amount of Joule heat is generated near the positive electrodecurrent collector 20 and this Joule heat induces an exothermic reactionbetween the positive electrode active material 24 and the non-aqueouselectrolytic solution.

Accordingly, the present inventors have considered that disposing areaction inhibitor 30 near the positive electrode current collector 20suppresses the reaction between the positive electrode 10 and thenon-aqueous electrolytic solution. The present inventors have alsoinvented to form a first mixture layer 22 containing the reactioninhibitor 30 on the positive electrode current collector 20 in order todispose the reaction inhibitor 30 near the positive electrode currentcollector 20. Further, if the reaction inhibitor 30 is present in thesurface portion of the positive electrode mixture layer 21, the reactioninhibitor 30 is considered to inhibit ion conduction duringcharging-discharging to deteriorate the input-output characteristics,and therefore the present inventors have invented to suppress thedeterioration of input-output characteristics by disposing a secondmixture layer 32 containing almost no reaction inhibitor 30 on the firstmixture layer 22 so that almost no reaction inhibitor 30 is present inthe surface portion of the positive electrode mixture layer 21. Althoughthe positive electrode mixture layer 21 has a two layer constitution ofthe first mixture layer 22 and the second mixture layer 32 in thepresent embodiment, the positive electrode mixture layer 21 may beconstituted of three or more layers as long as the concentration of thereaction inhibitor 30 contained in the positive electrode mixture layer21 near the positive electrode current collector 20 is higher than thatin the surface portion of the positive electrode mixture layer 21.

The first mixture layer 22 preferably contains 0.5% or more by mass and20% or less by mass of the reaction inhibitor 30 based on the totalamount of the positive electrode active material 24 as an amountexpected to exert the effect of the reaction inhibitor 30. A lower limitvalue of less than 0.5% by mass is not preferable because the effect ofthe reaction inhibitor 30 cannot be sufficiently obtained. Regarding theupper limit value, a value which makes it possible to ensure a desiredbattery capacity is preferable because the total amount of the positiveelectrode active material 24 is decreased as the added amount of thereaction inhibitor 30 is increased. Further, regarding the layerthickness of the first mixture layer 22, the value obtained by dividingthe layer thickness of the first mixture layer 22 by the sum of thelayer thickness of the first mixture layer 22 and the layer thickness ofthe second mixture layer 32 is preferably less than 0.5.

The second mixture layer 32 more preferably contains no reactioninhibitor 30 in order to suppress the deterioration of output-inputcharacteristics. As for a benchmark for containing no reaction inhibitor30, for example, 1% or less by mass is preferable, 0.5% or less by massis more preferable, and 0% by mass is particularly preferable based onthe total amount of the positive electrode active material 24 in thesecond mixture layer 32.

Although procedures for manufacturing the positive electrode 10 will bedescribed later, the interface between the first mixture layer 22 andthe second mixture layer 32 is considered to have a region, for example,in a range of approximately 10 μm in which the first mixture layer 22and the second mixture layer 32 are mixed together depending onconditions such as the particle diameter and the dispersibility of thepositive electrode active material 24, the electrically conductivematerial 26, the binder 28 and the reaction inhibitor 30, because thesecond mixture layer 32 is applied to the first mixture layer 22 andthereafter rolled. Accordingly, in order to discriminate between thefirst mixture layer 22 and the second mixture layer 32 from the crosssection of the positive electrode 10 as illustrated in FIG. 1, the mixedregion thereof needs to be considered.

[Negative Electrode]

For the negative electrode, any material which has been conventionallyused for the negative electrode in the non-aqueous electrolyte secondarybattery may be used without particular limitation. Such a negativeelectrode may be obtained by, for example, mixing a negative electrodeactive material and a binder in water or a suitable solvent, applyingthe resultant mixture to a negative electrode current collector, dryingthe applied material, and rolling the dried material.

The negative electrode active material is a material capable ofoccluding and releasing lithium ions. For such a negative electrodeactive material, there may be used, for example, carbon materials,metals, alloys, metal oxides, metal nitrides, and carbon and siliconpre-occluding lithium. The carbon materials include natural graphite,artificial graphite, pitch-based carbon fiber and the like. Specificexamples of the metals or alloys include lithium (Li), silicon (Si), tin(Sn), germanium (Ge), indium (In), gallium (Ga), lithium alloys, siliconalloys, and tin alloys. For the negative electrode active material,these materials may be used singly or in combinations of two or morethereof.

For the binder, a fluorine-containing polymer, an elastomeric polymer orthe like may be used, similarly to the case of the positive electrode10, but it is preferable to use styrene-butadiene copolymer (SBR), beingan elastomeric polymer, a modified material thereof or the like. Thebinder may be used together with a thickener such as carboxymethylcellulose (CMC).

For the negative electrode current collector, there is used a foil of ametal which forms almost no alloy with lithium in the potential range ofthe negative electrode, or a film disposed with a metal which formsalmost no alloy with lithium in the potential range of the negativeelectrode as a surface layer, or the like. For the metal which formsalmost no alloy with lithium in the potential range of the negativeelectrode, it is suitable to use copper, which is low cost, easy toprocess, and has good electron conductivity.

[Non-Aqueous Electrolyte]

The non-aqueous electrolyte contains a non-aqueous solvent and anelectrolyte salt dissolved in the non-aqueous solvent. The non-aqueouselectrolyte is not limited to a non-aqueous electrolytic solution, beinga liquid electrolyte, but may be a solid electrolyte.

For the non-aqueous solvent, there may be used cyclic carbonates, cycliccarboxylic acid esters, cyclic ethers, open-chain carbonates, open-chainesters, open-chain ethers, nitriles, amides and the like. Morespecifically, there may be used ethylene carbonate (EC) and the like forthe cyclic carbonates, γ-butylolactone (γ-GBL) and the like for thecyclic carboxylic acid esters, ethylmethyl carbonate (EMC), dimethylcarbonate (DMC) and the like for the open-chain esters. In addition,there may be used halogen-substituted substances which are formed bysubstituting a hydrogen atom of these respective non-aqueous solventswith a halogen atom such as a fluorine atom. Among others, it ispreferred to mix EC as a cyclic carbonate which is a solvent with a highdielectric constant, and EMC and DMC as open-chain carbonates which aresolvents with a low viscosity, and use the mixture.

As an electrolyte salt, a lithium salt generally used as a supportingelectrolyte for a non-aqueous electrolyte secondary battery may be used.As the lithium salts, there may be used LiPF₆, LiBF₄, LiClO₄ and thelike. These lithium salts may be used singly or in combinations of twoor more thereof.

In addition, the non-aqueous electrolyte may contain an additive usedfor the purpose of forming a coating that has excellent ion conductivityon the positive electrode or the negative electrode or the like. For theadditive, there may be used vinylene carbonate (VC), ethylene sulfite(ES), cyclohexylbenzene (CHB), modified substances thereof and the like.These additives may be used singly or in combinations of two or morethereof. The percentage of additive in the non-aqueous electrolyte isnot particularly limited, but is suitably approximately 0.05 to 10% bymass based on the total amount of the non-aqueous electrolyte.

[Separator]

For the separator, for example, there is used a porous film having ionpermeability and insulating properties disposed between the positiveelectrode and the negative electrode. The porous film may includemicroporous thin films, woven fabric, non-woven fabric and the like. Apolyolefin is preferably used as the material for the separator, andmore specifically polyethylene (PE), polypropylene (PP) or the like issuitable.

EXAMPLES

Hereinafter, the present invention will be more specifically illustratedin detail, referring to Examples and Comparative Examples, but thepresent invention is not intended to be limited to the Examples below.In the following examples, non-aqueous electrolyte secondary batteriesused in Examples 1-3 and Comparative Examples 1-6 were manufactured inorder to evaluate the effects of disposing the first mixture layer 22near the positive electrode current collector 20. The specificprocedures for manufacturing the non-aqueous electrolyte secondarybatteries are as follows.

Example 1

[Preparation of Positive Electrode]

A lithium-containing transition metal oxide represented by thecomposition formula LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ was used for a positiveelectrode active material 24. A positive electrode 10 was prepared asfollows. First the positive electrode active material 24 represented byLiNi_(0.5)Co_(0.2)Mn_(0.3)O₂, acetylene black as the electricallyconductive material 26, and polyvinylidene fluoride powder as the binder28 were mixed together so that the respective contents were 92% by mass,5% by mass and 3% by mass to give a mixture. Lithium fluoride (LiF) asthe reaction inhibitor 30 was mixed with the mixture at 5.5% by massbased on the mixture, and the resultant mixture was further mixed withan N-methyl-2-pyrrolidone (NMP) solution to prepare a positive electrodeslurry 1. This positive electrode slurry 1 was applied to both surfacesof the positive electrode current collector 20 made of aluminum having athickness of 15 μm by the doctor blade method, and was then dried andcompressed using a compression roller to form the first mixture layers22 on both surfaces.

Next, the positive electrode active material 24 represented byLiNi_(0.5)Co_(0.2)Mn_(0.3)O₂, acetylene black as the electricallyconductive material 26, and polyvinylidene fluoride powder as the binder28 were mixed together so that the respective contents were 92% by mass,5% by mass and 3% by mass, and the resultant mixture was further mixedwith an N-methyl-2-pyrrolidone (NMP) solution to prepare a positiveelectrode slurry 2. This positive electrode slurry 2 was applied to thefirst mixture layer 22 by the doctor blade method in an applied weightapproximately three times as much as that applied to the first mixturelayer 22, and then dried and compressed using a compression roller toform the first mixture layers 22 and the second mixture layers 32 onboth surfaces. Then, the thicknesses of the first mixture layer 22 andthe second mixture layer 32 were set to approximately 20 μm andapproximately 60 μm, respectively, and the relation between the layerthicknesses of the first mixture layer 22 and the second mixture layer32 was set to layer thickness of first mixture layer 22:layer thicknessof second mixture layer 32=1:3 (i.e., the value obtained by dividing thelayer thickness of the first mixture layer 22 by the sum of the layerthickness of the first mixture layer 22 and the layer thickness of thesecond mixture layer 32 was 0.25). Thus was obtained the positiveelectrode 10 formed of the positive electrode current collector 20, thefirst mixture layer 22 and the second mixture layer 32 stacked in thisorder.

[Preparation of Negative Electrode]

For the negative electrode active material, three kinds of materials ofnatural graphite, artificial graphite, and artificial graphitesurface-coated with amorphous carbon were prepared, and a blend thereofwas used. The negative electrode 42 was prepared as follows. At first,the negative electrode active material, styrene-butadiene copolymer(SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener weremixed together so that the respective contents were 98% by mass, 1% bymass and 1% by mass to give a mixture, the mixture was mixed with waterto prepare slurry, and then this slurry was applied to both surfaces ofa negative electrode current collector made of copper having a thicknessof 10 μm by the doctor blade method to form negative electrode activematerial layers. Then, the layers were compressed using a compressionroller to a predetermined density to give a negative electrode 42.

[Preparation of Non-Aqueous Electrolyte]

LiPF₆ was dissolved as the electrolyte salt at a concentration of 1.0mol/L in a non-aqueous solvent which had been prepared by mixingethylene carbonate (EC), ethylmethyl carbonate (EMC) and dimethylcarbonate (DMC) at a volume ratio of 3:3:4 to prepare a non-aqueouselectrolytic solution, and the solution was subjected to manufacture ofthe battery.

[Manufacture of Cylindrical Non-Aqueous Electrolyte Secondary Battery]

Further, the positive electrode, the negative electrode, and thenon-aqueous electrolytic solution thus prepared were used to manufacturea cylindrical non-aqueous electrolyte secondary battery (hereinafter,referred to as cylindrical battery) by the following procedures. Amicroporous film made of a polypropylene was used as the separator. FIG.2 is a perspective view showing a cylindrical battery 60 cutaway in alongitudinal direction. The positive electrode 10 prepared as describedabove was shaped in a size of short sides of 55 mm and long sides of 450mm and a collector tab 66 made of aluminum was formed at the centerportion in the long side direction of the positive electrode 10.Further, the negative electrode 42 was shaped in a size of short sidesof 57 mm and long sides of 550 mm and a collector tab 66 made of copperwas formed at each of the edge portions in the long side direction ofthe negative electrode 42.

The positive electrode 10 and the negative electrode 42 were wound witha separator having a three layer structure of PP/PE/PP interposedtherebetween to prepare a wound electrode body 46. Subsequently, thiswound electrode body 46 was disposed with insulation plates 62 and 63 onthe top and bottom respectively, and was accommodated in a cylindricalbattery outer can 50 made of steel having a diameter of 18 mm and aheight of 65 mm, the cylindrical battery outer can 50 also serving as anegative electrode terminal of the battery. Then, the two collector tabs64 of the negative electrode 42 were welded to the inner bottom part ofthe battery outer can 50 and the collector tab 66 of the positiveelectrode 10 was welded to the bottom plate part of the currentinterruption sealing body 68 provided with a safety valve and a currentinterruption device. The non-aqueous electrolytic solution was suppliedfrom the opening of the battery outer can 50, and then the battery outercan 50 was sealed with the current interruption sealing body 68 to givea cylindrical battery 60. It is noted that the cylindrical battery 60setup was done so that the rated capacity was 1200 mAh.

Comparative Example 1

A cylindrical battery for use in Comparative Example 1 was manufacturedin the same manner as that for Example 1, except that the positiveelectrode slurry 2 prepared in Example 1 was applied to both surfaces ofthe positive electrode current collector 20 by the doctor blade method,and was then dried and only second mixture layers 32 with a thickness ofapproximately 80 μm, to which lithium fluoride was not added as thereaction inhibitor 30, were formed on both surfaces of the positiveelectrode current collector 20.

Comparative Example 2

A cylindrical battery for use in Comparative Example 2 was manufacturedin the same manner as that for Example 1, except that the positiveelectrode slurry 1 prepared in Example 1 was applied to both surfaces ofthe positive electrode current collector 20 by the doctor blade method,and was then dried, and first mixture layers 22 with a thickness ofapproximately 80 μm were formed on both surfaces of the positiveelectrode current collector 20 and a second mixture layer 32 was notformed.

Comparative Example 3

A cylindrical battery for use in Comparative Example 3 was manufacturedin the same manner as that for Example 1, except that a slurry preparedby mixing 1.3% by mass of lithium fluoride as the reaction inhibitor 30in the positive electrode slurry 1 in Example 1 was applied to bothsurfaces of the positive electrode current collector 20 by the doctorblade method, and was then dried and first mixture layers 22 with athickness of approximately 80 μm were formed on both surfaces of thepositive electrode current collector 20 and a second mixture layer 32was not formed.

Example 2

A slurry prepared by mixing 3% by mass of melamine polyphosphate as thereaction inhibitor 30 instead of lithium fluoride in the positiveelectrode slurry 1 in Example 1 was applied to both surfaces of thepositive electrode current collector 20 by the doctor blade method, andwas then dried and first mixture layers 22 were formed on both surfacesof the positive electrode current collector 20 using a compressionroller, the positive electrode slurry 2 in Example 1 was applied to thefirst mixture layer 22 by the doctor blade method in an applied weightapproximately twice that applied in forming the first mixture layer 22,and dried to form a second mixture layer 32. Thereafter, a cylindricalbattery for use in Example 2 was manufactured in the same manner as thatfor Example 1, except that the stacked body formed of the positiveelectrode current collector 20, the first mixture layer 22 and thesecond mixture layer 32 stacked in this order was compressed using acompression roller to give a positive electrode 10 in which thethickness of the first mixture layer 22 was 25 μm and the thickness ofthe second mixture layer was 45 μm.

Comparative Example 4

A cylindrical battery for use in Comparative Example 4 was manufacturedin the same manner as that for Example 2, except that the positiveelectrode slurry 1 prepared in Example 2 was applied to both surfaces ofthe positive electrode current collector 20 by the doctor blade methodin an applied weight approximately 3.1 times that applied in forming thefirst mixture layer 22 in Example 2, then dried, and first mixturelayers 22 with a thickness of approximately 80 μm were formed on bothsurfaces of the positive electrode current collector 20 using acompression roller and a second mixture layer 32 was not formed.

Example 3

A slurry prepared by mixing 20% by mass of an aromatic phosphate esteras the reaction inhibitor 30 instead of lithium fluoride in the positiveelectrode slurry 1 in Example 1 was applied to both surfaces of thepositive electrode current collector 20 by the doctor blade method, andwas then dried, and first mixture layers 22 were formed on both surfacesof the positive electrode current collector 20 using a compressionroller. The positive electrode slurry 2 in Example 1 was applied to thefirst mixture layer 22 by the doctor blade method in an applied weightapproximately twice that applied in forming the first mixture layer 22,and dried to form a second mixture layer 32. Thereafter, a cylindricalbattery for use in Example 3 was manufactured in the same manner as thatfor Example 1, except that the stacked body formed of the positiveelectrode current collector 20, the first mixture layer 22 and thesecond mixture layer 32 stacked in this order was compressed using acompression roller to give a positive electrode 10 in which thethickness of the first mixture layer 22 was 15 μm and the thickness ofthe second mixture layer 32 was 60 μm.

Comparative Example 5

A cylindrical battery for use in Comparative Example 5 was manufacturedin the same manner as that for Example 3, except that the positiveelectrode slurry 1 prepared in Example 3 was applied to both surfaces ofthe positive electrode current collector 20 by the doctor blade methodin an applied weight approximately 3.1 times that applied in forming thefirst mixture layer 22 in Example 3, then dried, and first mixturelayers 22 with a thickness of approximately 75 μm were formed on bothsurfaces of the positive electrode current collector 20 using acompression roller and a second mixture layer 32 was not formed.

Comparative Example 6

A positive electrode slurry 2 containing no reaction inhibitor used inExample 3 was applied to both surfaces of the positive electrode currentcollector 20 by the doctor blade method, then dried, and first mixturelayers 22 were formed on both surfaces of the positive electrode currentcollector 20 using a compression roller. Then the positive electrodeslurry 1 containing an aromatic phosphate ester as the reactioninhibitor 30 used in Example 3 was applied to the first mixture layer 22by the doctor blade method in an applied weight approximately ¼ of thatapplied in forming the first mixture layer 22, and dried to form asecond mixture layer 32 containing the reaction inhibitor. Thereafter, acylindrical battery for use in Comparative Example 6 was manufactured inthe same manner as that for Example 3, except that the stacked bodyformed of the positive electrode current collector 20, the first mixturelayer 22 and the second mixture layer 32 stacked in this order wascompressed using a compression roller to give a positive electrode 10 inwhich the thickness of the first mixture layer 22 was 60 μm and thethickness of the second mixture layer was 10 μm.

[Evaluation of Discharge Capacity]

A charge-discharge test was performed at an environmental temperature of25° C. for the purpose of evaluating discharge capacities in Examples1-3 and Comparative Examples 1-6. The test method is as follows. Each ofthe cylindrical batteries was charged at a constant current of 1 C (1200mA) until the cell voltage became 4.2 V, and then continuously chargedat a constant voltage until the current value became 0.05 C (60 mA).Subsequently, each battery was discharged at a constant current of 1 C(1200 mA) until the cell voltage became 2.5 V, and further discharged ata constant current of ⅓ C (400 mA) until the cell voltage became 2.5 V.Table 1 shows the results of the sum of the discharge capacities at 1 Cand ⅓ C.

[Evaluation of Output Characteristics]

A charge-discharge test was performed at an environmental temperature of25° C. for the purpose of evaluating output characteristics of Examples1-3 and Comparative Examples 1-6. First, each of the cylindricalbatteries was charged at a constant current of 1 C (1200 mA) until thecell voltage became 4.2 V, and then continuously charged at a constantvoltage until the current value became 0.05 C (60 mA). Thereafter, eachbattery was discharged at a constant current of 1 C (1200 mA) to 50% ofthe discharge capacity of the cylindrical battery determined in theabove discharge capacity evaluation. Subsequently, each of thecylindrical batteries was discharged for 10 sec at constant currents of1 A, 5 A, 10 A, 15 A, 20 A, 25 A, 30 A, 35 A and 40 A, respectively, andthe cell voltages after 10 sec for respective discharge currents weremeasured. Then, the output characteristics were evaluated by reading thecurrent value at which the cell voltage became 2.7 V. Table 1 shows theresults.

TABLE 1 Positive electrode mixture layer Added quantity of 1st 2ndreaction mixture mixture Input-output characteristics inhibitor layerlayer Discharge Output Material for (% by thickness thickness capacitycurrent reaction mass) (μm) (μm) (mAh) (A) inhibitor Example 1 5.5 20 601201 35 LiF Comparative None None 80 1215 36 — Example 1 Comparative 5.580 None 1150 32 LiF Example 2 Comparative 1.3 80 None 1203 34 LiFExample 3 Example 2 3 25 45 1210 36 Melamine polyphosphate Comparative 380 None 1202 28 Melamine Example 4 polyphosphate Example 3 20 15 60 120535 Aromatic phosphate ester Comparative 20 75 None 1159 15 AromaticExample 5 phosphate ester Comparative 20 60 15 1198 22 Aromatic Example6 phosphate ester

As shown in Table 1, the discharge capacity in Example 1 and ComparativeExample 3 was approximately 1% less than that in Comparative Example 1,while in the case of Comparative Example 2, there was approximately 5%reduction. Further, the output characteristics in Example 1 were nearlyequivalent to those in Comparative Example 1, while Comparative Examples2 and 3, in which the reaction inhibitor 30 was contained almosthomogenously in the first mixture layer 21, resulted in a reducedoutput. The proportion of the reaction inhibitor 30 with respect to thetotal amount of the positive electrode active material 24 in Example 1was approximately 1.3%, which was almost the same as in ComparativeExample 3, and accordingly it was confirmed that Example 1, in which theconcentration of the reaction inhibitor 30 was set to be high near thepositive electrode current collector 20, did not affect input-outputcharacteristics such as discharge capacity and output characteristics.

The output characteristics in Examples 1-3 were equivalent to that inComparative Example 1, while Comparative Examples 2-6, in which thereaction inhibitor 30 was contained almost homogenously in the positiveelectrode first mixture layer 21, resulted in a reduced output.Comparative Example 6, in which the reaction inhibitor 30 was notcontained in the positive electrode first mixture layer 21 but wascontained in the second mixture layer 32, also resulted in deterioratedoutput characteristics.

[Nail Penetration Test]

A nail penetration test was performed for each of the cylindricalbatteries in a full-charged state in Examples 1-3 and ComparativeExamples 1-6 for the purpose of understanding the reaction-suppressingeffect by forming the first mixture layer 22 on the positive electrodecurrent collector 20. The test method is as follows. First, each of thecylindrical batteries was charged at a constant current of 1.0 C (1200mA) at an environmental temperature of 25° C. until the cell voltagebecame 4.2 V, and then continuously charged at a constant voltage untilthe current value became 0.05 C (60 mA). Subsequently, the centerportion in the side surface of each of the cylindrical batteries wascontacted with the tip of a wire nail with a thickness of 3 mmφ having asharp tip under an environment in which the battery temperature was 65°C., and the wire nail was pushed in along the diameter direction of eachof the cylindrical batteries at a velocity of 80 mm/sec, and when thewire nail completely penetrated each of the cylindrical batteries, thepushing in of the wire nail was stopped. Then, the behavior of thebattery temperature after pushing in was measured by contacting athermocouple on the battery surface. As battery temperatures, thebattery temperatures 5 and 10 sec after the pushing in and the maximumattained temperature were evaluated. The results of battery temperaturesare shown in Tables 2-1, 2-2 and 2-3.

TABLE 2-1 Battery temperature (a value within a parenthesis is adifference from Comparative Example 1) Maximum attained After 5 secAfter 10 sec temperature (° C.) (° C.) (° C.) Example 1 216 303 325(−80) (−60) (−47) Comparative 296 364 372 Example 1 (—) (—) (—)Comparative 228 299 320 Example 2 (−68) (−65) (−52) Comparative 309 319327 Example 3 (+13) (−44) (−45)

TABLE 2-2 Battery temperature (a value within a parenthesis is adifference from Comparative Example 4) Maximum attained After 5 secAfter 10 sec temperature (° C.) (° C.) (° C.) Example 2 237 295 299(−59) (−46) (−44) Comparative 296 341 343 Example 4 (—) (—) (—)

TABLE 2-3 Battery temperature (a value within a parenthesis is adifference from Comparative Example 5) Maximum attained After 5 secAfter 10 sec temperature (° C.) (° C.) (° C.) Example 3 261 291 295 (+2)(−Absorption (−54) layer 30) Comparative 259 321 349 Example 5 (—) (—)(—) Comparative 290 340 355 Example 6 (+31) (+19) (+6)

As seen from Table 2-1, Example 1 resulted in lower battery temperaturesafter 5 sec and 10 sec than those in Comparative Example 1, as well as alower maximum attained temperature. The time at which the batterytemperature reached the maximum attained temperature was after 10 sec inall of Example 1 and Comparative Examples 1-3. In particular, it can beseen that in Example 1 the battery temperature was lower than or almostequivalent to those in Comparative Examples 1-3 both after 5 sec and 10sec in the process of battery temperature elevation, and therefore theheat generation in the battery was suppressed. From this fact, it can beconsidered that, after heat generation due to a short circuit betweenthe positive electrode current collector and the negative electrodeactive material, the reaction inhibitor 30 present near the positiveelectrode current collector 20 suppressed the reaction between thepositive electrode active material and the non-aqueous electrolyticsolution induced by this heat and therefore could suppress the heatgeneration in the battery. In addition, even compared with the batteriesin Comparative Examples 2-3, in which a positive electrode 10 having nosecond mixture layer 22 and consisting of only a first mixture layer 22in which a reaction inhibitor 30 was almost homogeneously present in thepositive electrode mixture layer 21 was used, the battery in Example 1had an effect of significantly suppressing the elevation of the batterytemperature.

Furthermore, as shown in Table 2-2 and Table 2-3, in both of Example 2and Example 3, in which a melamine polyphosphate and an aromaticphosphate ester were used as a reaction inhibitor material,respectively, an effect of suppressing the elevation of the batterytemperature could be confirmed in the process of battery temperatureelevation even compared with the batteries in Comparative Example 4 andComparative Example 5. In addition, a high effect of suppressing theelevation of the battery temperature could be confirmed in Example 3even compared with a configuration with a reaction inhibitor containedin the separator side of the positive electrode active material such asComparative Example 6.

Thus, a positive electrode for a non-aqueous electrolyte secondarybattery wherein the concentration of a reaction inhibitor contained inthe positive electrode mixture layer near the positive electrode currentcollector is higher than that in the surface layer portion of thepositive electrode mixture layer, and a non-aqueous electrolytesecondary battery comprising this positive electrode for a non-aqueouselectrolyte secondary battery, suppress the reaction between thepositive electrode active material and the non-aqueous electrolyticsolution due to heat generation in an internal short circuit caused by anail penetration or the like, and have excellent input-outputcharacteristics.

REFERENCE SIGNS LIST

-   -   10 positive electrode, 20 positive electrode current collector,        21 positive electrode mixture layer, 22 first mixture layer, 24        positive electrode active material, 26 electrically conductive        material, 28 binder, 30 reaction inhibitor, 32 second mixture        layer, 42 negative electrode, 44 separator, 46 wound electrode        body, 50 battery outer can, 60 cylindrical battery, 62, 63        insulation plate, 64 negative electrode current collector tab,        66 positive electrode current collector tab, 68 current        interruption sealing body.

1. A positive electrode for use in a non-aqueous electrolyte secondarybattery, the positive electrode comprising a positive electrode currentcollector and a positive electrode mixture layer formed on the positiveelectrode current collector, the positive electrode mixture layercomprising a first mixture layer containing a positive electrode activematerial and a reaction inhibitor to suppress a thermal reaction betweenthe positive electrode active material and a non-aqueous electrolyte,and a second mixture layer comprising the positive electrode activematerial, the positive electrode formed of the positive electrodecurrent collector, the first mixture layer and the second mixture layerstacked in this order, wherein a concentration of the reaction inhibitorcontained in the positive electrode mixture layer near the positiveelectrode current collector is higher than that in a surface layerportion of the positive electrode mixture layer, the reaction inhibitorbeing in the form of at least one of powder and particulate that aresubstantially insoluble in the non-aqueous electrolyte.
 2. The positiveelectrode for a non-aqueous electrolyte secondary battery according toclaim 1, wherein the reaction inhibitor is at least one selected fromthe group consisting of lithium fluoride, aluminum fluoride, and afluoride.
 3. The positive electrode for a non-aqueous electrolytesecondary battery according to claim 1, wherein the reaction inhibitoris contained at 0.5% or more by mass and 20% or less by mass based on atotal amount of the positive electrode active material in the firstmixture layer.
 4. The positive electrode for a non-aqueous electrolytesecondary battery according to claim 1, wherein a value obtained bydividing a layer thickness of the first mixture layer by a sum of thelayer thickness of the first mixture layer and a layer thickness of thesecond mixture layer is less than 0.5.
 5. A non-aqueous electrolytesecondary battery comprising a positive electrode, a negative electrodeand a non-aqueous electrolyte, the positive electrode comprising apositive electrode current collector and a positive electrode mixturelayer formed on the positive electrode current collector, the positiveelectrode mixture layer comprising a first mixture layer containing apositive electrode active material and a reaction inhibitor to suppressa thermal reaction between the positive electrode active material andthe non-aqueous electrolyte, and a second mixture layer comprising thepositive electrode active material, the positive electrode formed of thepositive electrode current collector, the first mixture layer and thesecond mixture layer stacked in this order, wherein a concentration ofthe reaction inhibitor contained in the positive electrode mixture layernear the positive electrode current collector is higher than that in asurface layer portion of the positive electrode mixture layer, thereaction inhibitor being at least one selected from the group consistingof a phosphate ester, a melamine-acid salt, and a fluoride, the reactioninhibitor being in the form of at least one of powder and particulatethat are substantially insoluble in the non-aqueous electrolyte.
 6. Thenon-aqueous electrolyte secondary battery according to claim 5, whereinthe reaction inhibitor is contained at 0.5% or more by mass and 20% orless by mass based on a total amount of the positive electrode activematerial in the first mixture layer.
 7. The non-aqueous electrolytesecondary battery according to claim 5, wherein a value obtained bydividing a layer thickness of the first mixture layer by a sum of thelayer thickness of the first mixture layer and a layer thickness of thesecond mixture layer is less than 0.5.
 8. The non-aqueous electrolytesecondary battery according to claim 5, wherein the reaction inhibitoris at least one selected from the group consisting of lithium fluoride,aluminum fluoride, an aromatic condensed phosphate ester, melaminepyrophosphate, melamine sulfate, melamine cyanurate, and melamineborate.